Tape drives, such as, for example, digital data storage (DDS) tape drives are commonly used to back up and record information in the form of digital data from a computer system. A DDS tape drive, for example, is a helical scanning tape drive that writes and reads information in the form of digital data to and from a magnetic tape. DDS tape drives provide a low cost storage mechanism that is light weight, compact and typically very reliable. DDS tape drives have continued to evolve over time, such that each new generation of DDS drives and DDS formatted tapes provides additional storage capacity over the earlier generations.
One advantage of DDS tape drives over linear tape drives is the use of a helical scanning method that allows very high recording densities. Basically, a helical scanning method, such as, for example, in a DDS tape drive, uses a rapidly rotating scanner that has two read heads and two write heads (i.e., for a total of four transducers). The rotating scanner is tilted at an angle in relation to the horizontal movement of the tape, which is being transported at a given speed, and the tape is wrapped about at least a portion of the scanner. Thus, the horizontal tape movement against the tilted and rotating scanner causes diagonally positioned tracks to be written to the tape (and subsequently read from the tape). The speed and tension of the tape are typically kept constant by a tape drive servoing system that includes controlling circuitry and mechanical mechanisms, such as, for example, a capstan and series of rollers and guides.
The format of the recorded data tracks (e.g., containing raw, compressed, timing, control, and/or error correction data) on a DDS formatted tape, for example, is mandated by a specific DDS standard. The DDS2 format, for example, defines that the data tracks are to be written in groups of at least 23 tracks each.
During a read operation of a DDS formatted tape, for example, it is essential to align the read heads to the tracks as written in each of the groups during the previous write operation(s) so as to read the data within each of the groups. This is typically accomplished by adjusting the speed at which the tape is transported to properly align the read heads of the scanner to the previously recorded tracks.
However, since tapes are often used for backing up data and are capable of holding a fairly substantial amount of data, a single tape cartridge is written to several times before filling up. For example, in a DDS2 formatted tape, at least one additional group is added or appended to the tape during each subsequent write operation. It is also common to reuse previously recorded sections of the tape by writing a new group over all or part of an older group or groups, and/or to move a tape between different tape drives.
The result is that there can be several groups recorded on a given tape. Since each of the groups may have been written at a different time and/or at by a different tape drive, the positioning of the data tracks from group to group may be slightly different. For example, in accordance with the DDS2 specification there can be a vertical offset between groups of approximately 6.6 microns.
Thus, a tape drive needs to be configured to detect when the track positioning changes from one group to the next, else the tape drive will not properly read the tracks within each group. These changes typically present a timing issue that requires realignment of the read head(s) to account for differing track positions.